The present invention relates to the processing of sewage sludge and pertains particularly to improved method and apparatus for processing municipal sewage sludge for eliminating pathogens.
Densely populated areas of the world, such as cities, produce waste collected and treated as sewage that must be disposed of in a reasonably sanitary manner in order to protect public health. Most cities have sewer systems which collect and carry raw sewage, flushed by water to collection and treatment plants. The raw sewage is normally treated at various levels including anaerobic and/or aerobic digestion and sludge resulting from the treatment is disposed of in various manners including in land fills. A small percentage of it is disposed of on agricultural land. Disposal of the sludge is a problem because it contains all of the disease organisms common to man. Raw sewage collected at sewage treatment plants is normally first subjected to a primary treatment by settling and screening to dispose of sand, grit and some soluble solids. The settled solids are undigested raw sludge and normally pumped to aerobic digester where the raw sludge is converted to an organically stable form. The settled sewage is subjected to secondary treatment, where microorganisms are introduced with an excess of oxygen utilizing aeration. This secondary treatment results in digestion of the settled solids, aerobically and/or anaerobically. The resulting solids and treatment additives are separated or settled out and the sludge is removed for further processing.
The material that settles out is digested to further stabilize organic matter in the sludge solids. This treated sludge is dewatered and the sludge is then typically disposed of by commercial processors who are paid by the treatment plant to dispose of the sludge. The commercial processors further treat the sludge by composting it so that it can then be used for fertilizer or soil amendment. This amounts to an enormous cost to the treatment plant.
In the past, much of the sludge has been used in land fills or has been dumped in the ocean. These means of disposal are no longer permitted because of health and environmental concerns. A considerable effort has been undertaken to provide appropriate means of disposing of sludge without harming the population or the environment. Many techniques have been proposed for treating raw sludge to allow for disposal, such as dewatering and subsequent incineration. However, incineration introduces its own problems, such as air pollution from the burning of the sludge.
Many efforts are underway to recover the sludge and use it as a resource for its nutrient value for land application as fertilizer or soil amendment. Proposals have been made, for example, for its use in desert land reclamation. However, pathogens in the sludge must be removed before its application in or near populated areas where it is likely to be contacted by humans or animals.
Present concerns for public health require that the sludge be further processed if it is to be used as a fertilizer or as a soil conditioner, such as in land rock reclamation projects. This further treatment is required because the typical sludge product of a treatment plant still contains substantial quantities of bacteria, viruses and other organisms. One method that has been proposed for eliminating this problem is that of pasteurization. However, this is a costly process requiring expensive plant and equipment and substantial amounts of energy to carry out. Heating the sludge to effectively pasteurize it requires that it be heated to a temperature of at least about 160.degree. F. and maintained at that temperature for a minimum of 30 minutes. This is typically carried out in a batch process in large holding tanks to insure that no short circuiting occurs. Once the sludge has been pasteurized, it is suitable for land fills and land reclamation and low grade fertilizer. However, such pasteurization in this manner is expensive and not cost effective.
Presently known processes for pasteurization, however, are batch processes and are enormously expensive to construct and operate. Pasteurization, however, can ultimately reduce some cost of disposal by reducing the expense of paying commercial disposal companies for disposal of the sludge. Pasteurization can turn the sludge into a resource making it sufficiently desirable that much of the cost can be recovered. However, the pasteurization costs must be sufficiently low to make the whole operation economical.
It is therefore desirable that an inexpensive and cost-effective pasteurization system and process be available.